perf stat: Add -o and --append options

This patch adds an option (-o) to save the output of perf stat into a
file. You could do this with perf record but not with perf stat.
Instead, you had to fiddle with stderr to save the counts into a
separate file.

The patch also adds the --append option so that results can be
concatenated into a single file across runs. Each run of the tool is
clearly separated by a comment line starting with a hash mark. The -A
option of perf record is already used by perf stat, so we only add a
long option.

$ perf stat -o res.txt date
$ cat res.txt

 Performance counter stats for 'date':

          0.791306 task-clock                #    0.668 CPUs utilized
                 2 context-switches          #    0.003 M/sec
                 0 CPU-migrations            #    0.000 M/sec
               197 page-faults               #    0.249 M/sec
           1878143 cycles                    #    2.373 GHz
   <not supported> stalled-cycles-frontend
   <not supported> stalled-cycles-backend
           1083367 instructions              #    0.58  insns per cycle
            193027 branches                  #  243.935 M/sec
              9014 branch-misses             #    4.67% of all branches

       0.001184746 seconds time elapsed

The option can be combined with -x to make the output file much easier
to parse.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20110815202233.GA18535@quad
Signed-off-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-stat.txt

@@ -94,6 +94,13 @@ an empty cgroup (monitor all the time) using, e.g., -G foo,,bar. Cgroups must ha
 corresponding events, i.e., they always refer to events defined earlier on the command
 line.
 
+-o file::
+-output file::
+Print the output into the designated file.
+
+--append::
+Append to the output file designated with the -o option. Ignored if -o is not specified.
+
 EXAMPLES
 --------